The present invention relates to a wireless packet communication method and system for transmitting packets between a base station and at least one radio or mobile terminal station.
A multiple access scheme in a conventional wireless communication system has been realized by a frequency division multiple access (FDMA), a time division multiple access (TDMA), a code division multiple access (CDMA), or a hybrid multiple access combining these multiple accesses.
A plurality of radio channels of divided different frequencies in the FDMA, of divided different time slots in the TDMA and of divided different spread codes in the CDMA are assigned to respective radio terminal stations to carry out the multiple connections, respectively.
At a base station, a frequency band allocated to this base station is in principle equally divided to provide a plurality of carriers. For example, a frequency band of 1 MHz is divided to provide about 40 FDMA/TDMA carriers with an interval of 25 kHz, or a frequency band of 20 MHz is divided to provide 4 CDMA carriers with an interval of 5 MHz.
A carrier frequency of a radio channel allocated to a radio terminal station by a base station is fixed in principle when the radio terminal station is in communication with the same base station. Namely, the same carrier frequency is sequentially used even when instantaneous changes of frequency characteristics in the radio path may occur due to the movement of the radio terminal station.
Conventional wireless packet communication methods using multicarrier transmission will be now described in brief.
The orthogonal frequency division multiplexing (OFDM) scheme that will be utilized in a digital terrestrial broadcasting or a high data rate wireless LAN uses a plurality of subcarriers for sending signals. Each subcarrier will be usually set to a transmission rate considered as a flat fading, such as about 10 ksps at outdoors environment. However, since the OFDM scheme is designed to send a down-link signal to one user by using all subcarriers, transmission efficiency in each radio terminal station will deteriorate due to some subcarriers with poor received conditions.
Performance evaluation of high rate packet transmission using multicarrier CDMA (MC-CDMA) is described in for example Sadayuki Abeta et. al., xe2x80x9cPerformance comparisons of coherent SC/DS-CDMA, MC/DS-CDMA, MC-CDMA on down-link broadband radio packet transmissionxe2x80x9d, Technical Report of IEICE, RCS99-130, October 1999. According to this MC-CDMA, although each spread spectrum (SS) subcarrier with a wide frequency band is susceptible to frequency selective fading, there is little variation in the receiving state of the subcarrier.
FIGS. 1 and 2 illustrate conventional methods for transmitting radio packets.
Shown in FIG. 1 is the most simple method wherein packets requesting the transmission are sequentially sent in the order of the transmission request. In the figure, reference numeral 10 denotes a base station transmitter, 11 transmission requesting packets, and 12 radio packets sent from the base station. Numerals described in the transmission requesting packets 11 indicate numbers of radio terminal stations to which the packets are to be transmitted. In the example of FIG. 1, the packet to be transmitted to a No. 3 radio terminal station is located at the front-end, and the packet to be transmitted to a No. 4 radio terminal station is located at the tail-end.
The base station sends the radio packets 12 to the respective radio terminal stations without multiplexing. Each radio terminal station judges whether the received radio packet is destined to it or not from the header of the received radio packet. If the destined radio terminal station correctly receives the radio packet, this terminal station acknowledges by returning a response to the base station. The base station may send again the radio packet to the destined terminal station if no acknowledgement (ACK) is received. If it is judged from the ACK information that a good radio channel is established between the radio terminal station and the base station, more effective transmission using the multilevel modulation may be executed with this radio terminal station at the next transmission (adaptive modulation scheme).
However, according to this conventional method of FIG. 1, the base station will assign any subcarrier to a radio terminal station even if it may provide a poor received condition at this radio terminal station. Namely, all the subcarriers including good and bad received condition subcarriers are assigned causing the whole transmission efficiency to decrease.
When using SS signals for transmission, transmission requesting packets are multiplexed and sent in parallel to a plurality of terminal stations as shown in FIG. 2. In the figure, reference numeral 20 denotes a base station transmitter, 21 transmission requesting packets, and 22 radio packets transmitted from the base station.
Since each radio packet 22 is multiplexed by using different spread code, no assignment depending upon the received conditions is executed. In MC-CDMA, since each subcarrier uses a wide frequency band to expect the path diversity effect, the difference of the received conditions of the subcarrier becomes generally small.
According to this conventional method of FIG. 2, since the frequency band of each subcarrier is wide, each subcarrier may include some portions providing poor receiving conditions to the respective radio terminal stations. Therefore, the whole transmission efficiency will be decreased as well as the conventional method of FIG. 1.
It is therefore an object of the present invention to provide a wireless packet communication method and system, whereby no subcarrier providing poor receiving conditions to a radio terminal station, which causes degradation of transmission efficiency, is assigned to the radio terminal station.
According to the present invention, a wireless packet communication system including at least one radio terminal station, and a base station for multiplexing packets to be transmitted using a plurality of subcarriers and for sending the multiplexed packets to the at least one radio terminal station, has a unit provided in the base station for simultaneously sending reference signals on the subcarriers with the same level to the at least one radio terminal station, a report unit provided in the at least one radio terminal station for reporting to the base station received conditions of the reference signal of each subcarrier or of each subband corresponding to a frequency band occupied by a plurality of subcarriers, and a unit provided in the base station for allocating the subcarrier or the subband to each of the packets to be transmitted depending upon the reported received conditions of the subcarrier or the subband, the allocation being performed in a transmission order of the packets.
Furthermore, according to the present invention, a communication method in a wireless packet communication system including at least one radio terminal station, and a base station for multiplexing packets to be transmitted using a plurality of subcarriers and for sending the multiplexed packets to the at least one radio terminal station has a step of simultaneously sending from the base station reference signals on the subcarriers with the same level to the at least one radio terminal station, a step of reporting from the at least one radio terminal station to the base station received conditions of the reference signal of each subcarrier or of each subband corresponding to a frequency band occupied by a plurality of subcarriers, and a step of allocating in the base station the subcarrier or the subband to each of the packets to be transmitted depending upon the reported received conditions of the subcarrier or the subband, the allocation being performed in a transmission order of the packets.
The subcarriers with strong received intensities at each radio terminal station is assigned to packets to be transmitted and thus the packets are multiplexed. Therefore, according to the present invention, down-link signal transmission with improved frequency efficiency in comparison with the conventional packet transmission which equally uses all the subcarriers can be attained. Also, if the received conditions is reported for every subband that corresponds to a frequency band occupied by a plurality of subcarriers, the amount of information for the report can be reduced.
It is preferred that the subcarriers have a transmission rate similar to a rate of a flat fading.
It is also preferred that the report unit includes a first unit for reporting whether the received conditions of the each subcarrier or the each subband is equal to or higher than a first received condition threshold A or not to the base station. In this case, preferably, the first unit includes a unit for reporting the received conditions of the each subcarrier or the each subband by a binary one bit representing that the received conditions is equal to or higher than the threshold A or the received conditions is lower than the threshold A.
It is preferred that the report unit includes a first unit for reporting whether the received conditions of each subcarrier or each subband is equal to or higher than a first received condition threshold A or not to the base station, and a second unit for reporting identifiers of each subcarrier or each subband having the received conditions equal to or lower than a second received condition threshold B that is lower than the first received condition threshold A to the base station. In this case, preferably the second unit reports the identifiers at a radio terminal station that is a destination of a first packet of the packets, and the first unit reports the received conditions in radio terminal stations that are destinations of second and following packets of the packets.
It is also preferred that the report unit includes a first unit for reporting whether the received conditions of each subcarrier or each subband is equal to or higher than a first received condition threshold A or not to the base station, and a third unit for reporting identifiers of each subcarrier or each subband having the received conditions equal to or higher than a third received condition threshold C that is higher than the first received condition threshold A to the base station.
In this case, it is also preferred that the first unit reports the received conditions in a radio terminal station that is a destination of a first packet of the packets, and the third unit reports the identifiers at radio terminal stations that are destinations of second and following packets of the packets.
It is further preferred that the report unit includes a variation report unit for reporting only variations in received conditions at each subcarrier or each subband to the base station. The received conditions will be represented in multi-state expressions. In this case, preferably, the variations in received conditions is represented by binary bits.
It is preferred that the variation report unit includes a unit for reporting that there is large variations to the base station when the variations in received conditions is larger than a predetermined level. Thus, amount of information to be reported can be reduced.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.